[IPv6]Re: [v6ops] Re: New draft: "The IPv6 Loopback Address Prefix"

[Mark Smith <[e-mail]>]

Hi Ole,

On Thu, 27 Nov 2025 at 00:18, Ole Trøan <[e-mail]> wrote:
>
> Mark,
>
> Since the loopback address is of link-local scope you can already do this with multiple loopback interfaces.
> ::1%loop0
> ::1%loop1
>

That's interesting.

One thing I think worth avoiding is having to use loopback addresses
with zone IDs to minimise the amount of typing since these addresses
may be commonly typed in manually.

This was partly the thinking behind me choosing 1::/32 and having 1::1
be the default assigned address. The other advantage was that it's
only one digit different than ::1, making the new loopback prefix easy
to remember.

If somebody had to type in zone IDs, then the problem is already
pretty much solved - loopback interfaces are required to have
link-local addresses with a /64 prefix length, so they could be used.
However I think the amount of effort Brian and Bob had to put into
getting RFC9844 over the line shows that avoiding zone IDs in
addresses is preferable when possible.

Regards,
Mark.


> Cheers,
> Ole
>
> > On 26 Nov 2025, at 11:34, Mark Smith <[e-mail]> wrote:
> >
> > Hi,
> >
> >> On Wed, 26 Nov 2025 at 20:06, Gert Doering <[e-mail]> wrote:
> >>
> >> Hi,
> >>
> >>> On Wed, Nov 26, 2025 at 07:20:17PM +1100, Mark Smith wrote:
> >>> On Wed, 26 Nov 2025 at 18:26, Gert Doering <[e-mail]> wrote:
> >>>> On Wed, Nov 26, 2025 at 11:03:20AM +1100, Mark Smith wrote:
> >>>>> On Wed, 26 Nov 2025 at 10:19, Geoff Huston <[e-mail]> wrote:
> >>>>>> thanks Mark - except that you should  substitute the /32 to a /96 as I got confused between my left and my right!
> >>>>>>
> >>>>>
> >>>>> One of my use cases, which I think I hinted to but didn't specifically
> >>>>> state, was multiple loopback interfaces on a node.
> >>>>>
> >>>>> On Cisco routers you can create multiple loopback interfaces. I've
> >>>>> done that so that I could have say a management loopback interface and
> >>>>> address, a BGP session loopback interface and address and then other
> >>>>> loopback interfaces and addresses for other functions such as an
> >>>>> L2TPv2 end point, RADIUS service client address etc.. Those loopback
> >>>>> interface addresses were all announced into the routing protocol (or
> >>>>> not depending on function).
> >>>>
> >>>> This is a very different thing from what the draft proposes to establish.
> >>>>
> >>>> You are basically deliberately choosing and assigning routable addresses
> >>>> to "non physical" device interfaces, for externally-reachable functions
> >>>> (and this is a very reasonable thing to do, so "all the router people"
> >>>> do it, and "all the anycast service people" do it as well...).
> >>>>
> >>>
> >>> Yes. I mentioned it to demonstrate that I think a larger loopback
> >>> prefix should be big enough to support multiple loopback interfaces
> >>> within a host.
> >>
> >> I find this very hard to understand.
> >>
> >> Why would "there is a larger loopback prefix, used only for machine-internal
> >> communication" have any relevance to "I use multiple loopback interfaces with
> >> manually configured routable IP addresses"?
> >>
> >> Why would "having multiple loopback interfaces out of the well-known
> >> machine-internal loopback address space" have any relevance to anything?
> >>
> >
> > It's in the context of the draft I wrote proposing a larger IPv6
> > prefix. The prefix should be large enough to support subnetting it so
> > that those sub-prefixes can be assigned to multiple loopback
> > interfaces on a host.
> >
> > A ::/96 prefix means having to subnet in an unfamiliar location within
> > the IPv6 address.
> >
> > A /48 for the loopback prefix would allow subnetting at the familiar
> > /48 boundary, resulting in familiar /64s sized subnets that are
> > assigned to different loopback interfaces on the host.
> >
> > Assignment of loopback prefixes to loopback interfaces should be
> > automated. I think that is one of the key values of 127.0.0.1/8 - it's
> > automatically configured. Using at least /48 and then a /64 per
> > loopback interface allows doing something like using the interface's
> > ifIndex as the /64 subnet number (ifindex are typical 16 bits in size,
> > same as the /48 through /64 bits).
> >
> > Loopback interfaces are required to have a link-local prefix per
> > rfc4291, and their prefix length is a /64. The drawback of using a
> > link-local address for testing over a loopback interface is that they
> > require a zone identifier.
> >
> > While a /48 would be fine, since it's better to try to avoid having to
> > grow the loopback space in the future (e.g. which is what has happened
> > with the documentation prefix), a /32 or 1 x 4 billionth of the IPv6
> > address space, for a larger loopback prefix seems worth while.
> >
> > Regards,
> > Mark.
> >
> >> Gert Doering
> >>        -- NetMaster
> >> --
> >> have you enabled IPv6 on something today...?
> >>
> >> SpaceNet AG                      Vorstand: Sebastian v. Bomhard,
> >>                                           Karin Schuler, Sebastian Cler
> >> Joseph-Dollinger-Bogen 14        Aufsichtsratsvors.: A. Grundner-Culemann
> >> D-80807 Muenchen                 HRB: 136055 (AG Muenchen)
> >> Tel: [phone]         USt-IdNr.: DE813185279
> >
> > _______________________________________________
> > v6ops mailing list -- [e-mail]
> > To unsubscribe send an email to [e-mail]